The human foot is bio-mechanically extremely complex and is anatomically quite sophisticated. The foot must absorb forces which during sports or other rigorous activities can apply forces to the foot which are four to five times the original body weight. It has been reported that 75%-80% of the people in the United States have some form of foot morbidity. These people are usually in moderate to severe pain, and a variety of treatments have been used.
Because most people wear shoes, efficient transfer of forces from the foot to the ground through the shoe is critical.
Anatomically, the bottom (plantar) surface of the human foot can be divided into four areas. These are, the rear or heel area, the mid-foot or arch area, the metatarsophalangeal joint or ball area, and the toes. The foot basically shows three plantar bony prominences which are the first metatarsal head, the fifth metatarsal head, and the heel. These are joined by a strong skin which traverses the concave area generally referred to as the "arch". This "arch" is the medial longitudinal arch, and the foot also comprises a lateral longitudinal arch which projects forward from just in front of the heel and a transverse metatarsal arch which extends from one side of the foot to the other in the ball area of the foot.
The human gait has been analyzed in some detail, and the ground-contact, or stance phase is generally considered to comprise four parts. These are the heel strike, the fore-foot loading, the heel lift, and the toe off. Proper foot functioning results in a smooth flow of forces from the body to the shoe and the ground throughout the entire phase.
When moving from heel strike to forefoot loading, the foot unlocks it's arch components and depresses to absorb shock and to adjust for uneven terrain. If the arch depresses excessively, propulsion, which carries the highest vertical force, will be initiated from an unlocked structure which will subject it to injury, such as a strain or a sprain. The biomechanical principle involved is that an unstable bone will not support weight but will instead be placed in motion by the forces acting on it. In addition, progressive deformities may result.
The arch depressing stage is referred to as "pronation." Many factors may cause pronation to excess, such as a high arch in a flexible foot, a hard walking surface, excessive weight, extremely tight calf muscles or poorly designed shoes. Many physicians believe that abnormal pronation of the foot is the most common cause of foot pathology today.
Many techniques have been proposed to treat problems with the foot in one way or another. U.S. Pat. No. 2,475,417 (Wysowski) teaches a triangular pad having a front part which extends directly from a location under the metatarsal bone of the large toe to a position at the other side of the foot under the metatarsal bone of the little toe. The third corner of the pad is substantially at the center of the foot
U.S. Pat. No. 1,673,126 (Sarra) also teaches a triangular pad for fitting in the arch of a foot.
U.S. Pat. No. 3,265,071 (Kirchner et al) teaches an orthopedic pad which fits only in a small region of the arch of a foot.
U.S. Pat. No. 3,997,984 (Hayward) teaches a canvas shoe having a raised portion in the sole which fits in the arch of the foot.
U.S. Pat. No. 4,316,333 (Rothchild) teaches removable foot support devices which fit in a variety of locations of a shoe.
U.S. Pat. No. 4,408,402 (Looney) shows a shoe also having an arch support, and U.S. Pat. No. 4,627,177 (Meyers) shows an insole having support which extends from the longitudinal arch to the heel.